A siege is an army that attacks and waits for the enemy to come out of the castle and surrender. The army surrounds the castle and can wait months or even years. The army attacks when the people behind the wall run out of food. The reason for this is because the people who are going to attack want the land and more power.
The Ballista shoots a really dangerous arrow and it can do some real damage. The Ballista is really heavy and really powerful. The Ballista works by shooting big arrows or little ones over the wall and destroys buildings and other things.
http://www.ancient fortresses.org/ballista.htm http://medieval lifestyle.com/siege-engines/ballista. html http://www.dictionary.com/browse/siege